The present invention deals with water getter devices for laser amplifiers and a process for the manufacture thereof.
Laser amplifiers are used in the field of telecommunications on optical waveguides. As is known, in such an application the signal is a light pulse traveling along a glass fiber. When traveling along the fiber, the optical signal undergoes attenuation, in consequence of which signal amplifications are needed at given distances, generally comprising between about 50 and 100 km, or at every branching of the line.
To this aim devices are used, which are called optical amplifiers, wherein the signal is brought back to the initial intensity. Generally optical amplifiers comprise electronic components and a laser device, for example a high power gallium arsenide (GaAs) laser, which is the active element in the amplification of an optical signal. The laser device is located in a container having the purpose of keeping under control the gaseous environment around the device itself to ensure its working. In particular, it is necessary to avoid the laser device coming into contact with humidity which could cause either short circuits in the electronics of the device or a corrosion of the materials it is made of.
Water can be present in the container as a residue of the manufacturing process; otherwise it can be produced within the amplifier during its working as a consequence of the reaction between oxygen and the hydrogen released by the walls of the amplifier; or it can even be generated by reaction between traces of organic compounds (such as alcohols or glues, which are utilized in producing the amplifiers) and oxygen. According to the teaching of the U.S. Pat. No. 5,392,305, oxygen is generally added on purpose to the filling atmosphere of the device (generally nitrogen or a rare gas) in a quantity of at least 100 ppm, just in order to remove traces of organic compounds which are known to have a detrimental effect on the amplifier life.
The use of water getter materials in laser amplifiers is described e.g. in the U.S. Pat. No. 5,696,785 that discloses a system for the sorption of various gases among which water, consisting of at least a natural or synthetic zeolite, a compound chosen among porous silica, activated carbon, alumina and some zeolites, and an organic agent with the function of binding the mentioned components. The mixture is compacted by using suitable thermal treatments, with formation of sheets not thicker than one millimeter, which are caused to adhere to one of the inner surfaces of the amplifier container. However the sorption system described in this patent is somewhat complex and there is not specified how it is fixed at the inside of the laser container. Furthermore, with this solution there is even the risk that powders or fragments of the getter material are susceptible of free movements within the laser amplifier container; such particles could interact chemically with or damage mechanically the various components of the amplifier or could also become interposed in an optical path within the device, thus damaging its operation in any case.
European patent application EP-A-720 260 discloses another system for the gas sorption in laser amplifiers. In this case the getter material, in the form of a pellet or a tablet such as according to the above mentioned U.S. Pat. No. 5,696,785 or in the form of loose powders, is provided to be held in a suitable housing and in contact with the gaseous atmosphere within the amplifier, only through a surface permeable to the gases but capable of retaining solid particles. Such a construction should prevent fragments or powders of the getter material from being capable of free movements in the container. The gas permeable surface (such as micro-perforated metal plate) is fixed to the other walls of the getter housing or directly to the wall of the laser device container by means of welding; the welding can be carried out in different ways, e.g., resistance welding or, due to the small sizes involved, laser welding.
To ensure that this system does not lose particles, it is required that the welding is a continuous weld (“continuous seam” welding). However, it is known that a welding with a length of some millimeters or tens of mm are hardly carried out without showing any discontinuity; these discontinuities are critical points for the water getter device, as described in application EP-A-720 260, since they can form ways of passage for the getter material power through which this can reach the inner space of the amplifier with the inconvenience mentioned above. Furthermore, with the “continuous seam” welding scarcely connected burrs are always produce which can detach thus giving the same problems as already discussed for the powers of the getter material.